In the oil and gas industry subsea wellbores are drilled from surface vessels, such as drill ships, semi-submersible rigs, jack-up rigs and the like, as is well known in the art. Typically, a drilling riser is provided which extends to span the water column between the wellhead and a surface vessel to provide a contained passage for equipment and fluids. To this extent the drilling riser normally includes a large bore central riser pipe which accommodates the drilling equipment and certain fluids, such as drilling fluids and wellbore fluids, and a number of auxiliary or peripheral conduits which extend alongside the central riser pipe and provide communication of control fluids, well kill fluids, choke fluids, hydraulic power fluid and the like. Such auxiliary lines may terminate at the wellhead, for example at a Blow Our Preventer (BOP). In some applications the central riser pipe may define an internal diameter of around 21 inches (53.34 cm), and the auxiliary lines may be in the range of 2-4 inches (5.08-10.16 cm) internal diameter.
The drilling riser is typically formed from a number of individual sections or joints which are secured together in end-to-end relation. Each individual joint includes the required auxiliary lines arranged around a length of riser pipe, wherein the ends of the riser pipe and auxiliary lines are terminated at opposing flange connectors. During deployment, the individual sections or joints are secured together via the flange connectors, with appropriate seals utilised.
The auxiliary lines are typically pressure rated to the shut-in pressure of the wellhead and consequently are thick walled and relatively heavy. To reduce the riser top tension needed to safely support the riser during operation external buoyancy is often added to each joint to reduce the in-water weight. However, this buoyancy has the disadvantage of increasing the joint outside diameter and also the in air weight, and the buoyancy adds to problems associated with riser joint topside handling and storage. It also presents problems during deployed operation adding significantly to the riser hydrodynamic drag.
The central pipe is extended up to the drill floor of the surface vessel whilst the auxiliary lines are terminated at an elevation below the drill floor, typically within the moonpool. Pressure rated fluid connections are made between the top of the auxiliary lines to piping headers on the vessel. These connections are achieved using flexible jumpers. These flexible jumpers accommodate the relative motion between the riser and vessel which may arise due to a combination of vertical heave and angular rotations. These flexible jumpers are critical items since they must be flexible and reliably accommodate the full wellhead pressure during periods of pressure testing and emergency well control.
To accommodate the relative vertical heave and angular motions, known flexible jumpers are configured in free hanging catenaries connected to steel goosenecks at the top end of each auxiliary line. In order to meet the spatial constraints of the moonpool the flexible jumpers typically must accommodate a small minimum bend radius, which is a significant design challenge. Also, high pressure ratings will require a significant steel content, thus resulting in high weight.
At the seabed end of the drilling riser, similar flexible jumpers are used to connect across a lower flex joint interfacing between the dynamic lowermost riser joint and a static Lower Marine Riser Package (LMRP). In this application the flexible jumpers typically only need to accommodate angular motions but must additionally accommodate the challenge of high hydrostatic pressures that can cause collapse of the jumpers, particularly when subjected to simultaneous bending.
These flexible jumpers are critical to well control procedures and the safe operation of the drilling system. However, experience shows that such jumpers often need to be replaced due to damage and they are difficult to handle because of their high weight and stiffness. A further limitation of these flexible jumpers is that they are limited in pressure rating and suffer from fatigue damage particularly when they are operated with high temperatures and with sour fluid conditions. This causes operational difficulties and downtime.
The auxiliary conduits also provide many challenges in terms of high weight, corrosion and internal fluid cleanliness and their poor reliability resulting from many seals between each joint. These seals need to be regularly replaced and during riser installation regular pressure testing is necessary which takes time and when failures are detected riser sections need to be pulled back taking additional time. However, currently there has been no practical solution to eliminating the need for these seals due to handling constraints. Consequently the drilling industry has overcome these problems by adding large qualities of buoyancy and rigorous procedures to inspect, maintain and replace seals and seal surfaces.